Telephone system



5, 1936. w. H. GRINSTED ET AL 2 9 TELEPHONE SYSTEM Filed Aug. 18, 1935 Fla] WILL/AM HERBERT GRl/VSTED (/AMES ERR/EAL TEES Patented May 5, 1936 UNITED STATES TELEPHONE SYSTEM William Herbert Grinsted, Bromley, and James Errigal Tees, London, England, assignors to Siemens Brothers & don, England Company Limited, Lon- Application August 18, 1933, Serial No. 685,710 In Great Britain August 25, 1932 Claims.

The present invention relates to telephone systems and is more particularly concerned with the supply of current for transmitter feeding purposes.

In present day telephone systems, the current to a subscribers transmitter is fed 'froma common battery at the exchange to which the subscribers line is connected, the feed being over windings of a relay or relays, or of a retardation coil, or of a repeater. In order that sufficient current to provide adequate transmission efficiency may be supplied over lines of several hundred ohms resistance, it has become general practice to use batteries having a voltage higher than waspreviously the practice, the voltage used ranging between 40 and 60 volts.

With these voltages the current which flows through the relay or coil windings to short subscriber-lines, or under fault conditions, may be great enough to cause overheating, unless the resistance to line of the feed circuit is above a given value. Thus a relationship exists between battery voltage and feed circuit resistance which is, to some extent, determined by regard to the safety of the feed coils and subscribers transmitter when used on short subscriber-lines.

In the current supply arrangement described in British Patent No. 372,004, telephone lines are fed with operating current through impedance coils of lower ohmic resistance than is required, with the battery voltage generaly used, to limit the current fed to the short lines to the required degree, and a ballast resistance is connected in series with the impedance coils, the characteristics of the ballast resistance being such that, while it limits the current fed to short lines, its limiting effect decreases as the line resistance increases, so that the difference between the current fed to short lines and that fed to long lines is not so great as it would be through the impedance coils of reduced resistance alone. In order to maintain at the desired level the impedance of the feed circuit to currents of speech frequency, the system referred to provides for the improvement of the magnetic circuit, for instance by the use of special magnetic material as described in British Patent No. 254,364.

In general, with such an arrangement there is a gain in decibels as regards the local line sending allowance. It may be, however, that a still greater gain in local line sending allowance is desirable. There is a limit, however, to the reduction in the ohmic resistance of the impedance coils that can be effected with satisfactory results, having regard to the necessity for maintaining the impedance of the coils to currents of speech frequency.

According to the invention the desiredgreater gain is obtained by connecting the feeding bridges to a source of current the potential of which is considerably higher than that normally used, one or more ballast resistances being used to limit the excessive current that would otherwise fiow in the case of short subscriber lines.

We may increase the voltage of the battery to double that now commonly used, that is We may increase it to volts.

When, as is at present usual, a 50 volt battery is used, it is the common practice to connect one pole, the positive, to earth, with the result'that the maximum pressure between any part of the system and earth is 50 volts. In order that,when a 100 volt source of currentis used, the maxi mum pressure between any part of the system and earth may still not exceed 50 volts, either positive or negative, we connect the centre point instead of one pole of the battery to earth.

It is usual in exchanges in which metering of calls is effected by means of a booster battery to In connections between subscriber-lines and junction lines, two types of connection are-involved, one in which the subscriber-line is calling, and the other in which it is the called line. Moreover, the conditions for the first type of con nection may be different, if the exchange on which the subscriber-line terminates is a main exchange, from the conditions if the exchange is a satellite exchange. If the exchange is a main exchange, the subscriber-line may be fed with current by a bridge in an outgoing repeater {I connected in the junction line, and since the repeater is used only on junction connections, the connection of the 100 volt battery to subscriber-lines only when they are calling for junction connections, presents no difficulty.

If on the other hand, the exchange is a satellite exchange, subscriber lines are normally fed, whether they are to be connected one to another, or to a main exchange subscriber-line, by a bridge in a so-called discriminating selector at the satellite exchange. In order to provide the increased voltage only when a subscriber-line on a satellite exchange is connected to a junction line, we may arrange that the feeding bridge associated with the discriminating selector is cut out when a. digit appropriate to a junction connection is dialled and provide in the junction an outgoing repeater with a special bridge adapted to feed current from the 100 volt battery to the calling side of the bridge. would thus be used only on junction connections. The feeding bridge associated with an outgoing repeater and a manner of connecting the ballast resistances into the A feed circuit, are illustrated in Fig. 1 of the accompanying drawing, in which the two 50 volt batteries are represented at +50 and --50, and the two ballast resistances at BL! and BL2. Relay L is the usual feeding relay or coils. In view, however, of the ballast resistances BL, the resistance of L is considerable reduced, for instance to 50+50 ohms. The junction is connected to the wires represented at the right of the figure, IL being a retardation coil and RR a relay used, for instance, for signalling purposes.

In the second type of connection, that is the type in which the subscriber-line is the called line, the current may be fed to the subscriberline from a feeding bridge in an incoming selector. Since this selector is used only for junction calls, there is no difficulty in providing it with a. feeding bridge of any desired character and the incorporation of the 100 volt ballast resistance presents no difliculty. Such a feeding bridge is illustrated in Fig. 2 of the accompanying drawing, in which BAI and BA2 are the ballast resistances and D the usual feeding relay. The resistance of D is reduced on account of BA.

We may use for the purpose of the present invention, two ballast resistances, each being in two portions, one portion in each being connected, in series, between one pole of the battery and one wire of the line, and the other portion in each being connected, in series, between the other pole of the battery and the other wire of the line. This method of connecting the ballast resistances BRI and BRZ is shown in Figi 3 of the accompanying drawing and has the advantage that balanced pairs of resistances are mechanically associated, BRI and BRZ having, it is understood, the same characteristics as have the ballast resistances in the arrangement of British Patent No. 372,004.

We claim:

1. In a telephone line talking bridge wherein a current source of one voltage is ordinarily em- The 100 volt battery,"

ployed in the bridge to serve subscribers lines of Widely varying electrical characteristics, the inclusion of an additional source of current potential to insure sufiicient current flow when the electrical characteristics of lines unfavorable to satisfactory operation when the first specified current only is employed, together with the inclusion of a ballast resistance for preventing an excessive current to flow when a line of favorable electrical characteristics to the first specified current value is connected to the bridge.

2. In a battery feed bridge, for supplying talking battery to lines of widely varying electrical characteristics, the provision of a source of current additional to that ordinarily included in the bridge, and ballast resistances included in circuit with the ordinarily included current source and in circuit with the additional current source, respectively, to compensate for the varying electrical characteristics of such lines and thereby maintain aconstant-current flow irrespective of which of such lines is connected with the bridge.

3. In a battery feed bridge, for common useby a plurality of subscribers lines of differing electrical characteristics, a source of current and a current flow regulating element included in and constituting a part of said bridge, said current flow regulating element having the inherent characteristic of varying its resistance. in inverse proportion to the resistance of the line across which the bridge is connected. v

4. A battery feed bridge, one leg of the bridge comprising a source of current, an impedance winding and one filament of each of two double filament ballast resistances in'series, the other leg comprising a second source of current, a second impedance winding and the other filament of each of the said two ballast resistances in series; said ballast resistances jointly functioning to maintain uniform transmission over connected lines of varying electrical characteristics.

5. In a telephone system wherein the battery feed for lines of varying electrical characteristics ordinarily comprises a source of current in circuit with the two windings of an impedance coil, the provision of an auxiliary source of current and of ballast resistance elements, the said auxiliary source of current serving to insure an ample current flow when the bridge is employed with a line of high resistance, while the ballast resistances serve to prevent an excess of current flow when lines of relatively low resistance are connected to the bridge.

WILLIAM HERBERT GRINSTED. JAMES ERRIGAL TEES. 

